What if i use an add on which alters the page output, for example :
An add on that adds warning message to forum header, with big red font, for guests only from country A / B.
Does guest page caching also consider that for the variations?
So guests from country A & B see the warning message, and guests from country C/D/E don't?
In general, no it can't account for things like that. If you do things that may cause the page to vary per guest, guest page caching is likely not appropriate for your use case.